A dry mortar blend is mixed with water to make a workable mortar paste which is useful as a common construction material to bind construction blocks together and fill the gaps between them. A mortar paste becomes hard when it sets, resulting in a rigid aggregate structure. Mortar can also be used to fix, or point, masonry when the original mortar has washed away.
Present-day dry mortar blends are typically made from a mixture of an aggregate such as sand, a binder such as cement or lime, and water. Commonly, a dry mortar blend consists of about 25% of cement and of about 75% of sand. Just before use, the dry mortar blend is mixed with water. The dry mortar blend is stored dry in bags.
Cement is thus used as the binder material that hardens to form the connecting material between solids. Cements that are used in construction are either hydraulic or non-hydraulic. Hydraulic cements (e.g., Portland cement) harden because of hydration, being a chemical reaction between anhydrous cement powder and water.
Consequently, they can harden underwater or when constantly exposed to wet weather. The chemical reaction results in hydrates that are not very water-soluble and so are quite durable in water. Non-hydraulic cements do not harden underwater. Slaked limes for example harden by reaction with atmospheric carbon dioxide.
Portland cement is by far the most common type of cement in general use around the world. This cement is made by heating limestone (calcium carbonate) with small quantities of other materials (such as clay) to a temperature of 1450° C. in a kiln, in a process known as calcination. In a calcination process, a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, also called quicklime, which is then blended with the other materials that have been included in the mix. The resulting hard substance, called ‘clinker’, is then ground with a small amount of gypsum into a powder to make ‘Ordinary Portland Cement’, the most commonly used type of cement (often referred to as OPC).
There exist different types of cement, indicated with CEM I to CEM V, with a lower or higher content of Portland cement and high oven cement, i.e.                CEM I: OPC with maximally 5% of other compounds;        CEM II: all kinds of mixtures of OPC and for instance slate, minimally 65% OPC;        CEM III: high oven/Portland cement mixture in 3 classes: A, B and C, wherein CEM III/A comprises the lowest (40%) and CEM III/C the highest (60%) amount of blast furnace slag;        CEM IV: types of Pozzolana cement;        CEM V: composite cements, with mixtures of OPC, blast furnace slag and Pozzolana compounds.        
Depending on the climate and the application, a setting accelerator is needed. For instance, in cold weather conditions, it can be beneficial to use a setting accelerator to have a setting time reduction to prevent undercooling and loss of strength and durability.
Until now, the most commonly used setting accelerator is calcium formate (Ca(HCOO)2). Calcium formate is a non-hygroscopic powder that shows good response with Ordinary Portland Cement (OPC). The disadvantage of calcium formate however is that it shows a weak response on the more and more popular blended cements like CEM II/A-V. In addition, calcium formate is quite an expensive calcium salt.
Calcium chloride (CaCl2), which is the most cost efficient calcium salt, can also be used as a setting accelerator. Mortar based structures however are often reinforced with metal pieces to keep the mortar in shape during production. In case of reinforcement, the disadvantage when using chlorides is that these chlorides are leading to metal corrosion, through which the expansion of the formed “rust” leads to volume expansion and cracking of the mortar. This has as a consequence that the durability of the mortar made reinforced element is reduced.
It is also possible to use lithium carbonate (Li2CO3). The disadvantage of using carbonate however is that it reduces the pH value of the mortar and that consequently, it can give harm to embedded reinforcement.
There are approaches to use hardening accelerators instead of setting accelerators. Common types are sodium thiocyanate, triisopropanolamine (TIPA) or triethanolamine (TEA). These substances lead to increased intensity of reactions what also results in earlier finishing of the setting period. However, as a matter of fact, those hardening accelerators are hazardous substances. Therefore, such substances can hardly be used with respect to the environment as well as with respect to the user thereof.
Calcium nitrate (Ca(NO3)2) is commonly used as a concrete admixture, usually in the form of a solution. The final liquid product for end users is prepared from either a solution or a dissolved powder, granules or prills. Calcium nitrate shows a good response on most blended cement types and is a common admixture for concrete. Usually 1% to 2% calcium nitrate needs to be added by weight of cement (=bwoc).
In WO 2002006182, it is described to use particulate cellulose material such as untreated rice husks to extend hydraulic cement compositions. In order to overcome the problem that the untreated rice husks do not bind well with wetted cement compositions, an accelerator or setting is added to the cement mixture. In the description, it is stated that good results are obtained with calcium nitrate. However, no further details are given in which form and what concentration the calcium nitrate is added to the cement.
Calcium nitrate powder is commonly found as a tetrahydrate. With more than 30% of crystal water, calcium nitrate salt is wet. As a result, when this tetrahydrate calcium nitrate powder is blended with cements, the blend clogs. Furthermore, calcium nitrate granules and prills cannot be directly used in mortar, since granules that are added to water, together with cement and sand, do not dissolve in the water because the sand and the cement consume the water and because of their low affinity towards water. The amount of remaining free water is too low to dissolve the calcium nitrate granules, and the concentration of calcium nitrate is too high so that it finally precipitates.
The calcium ammonium nitrate complex salt (ammonium nitrate:calcium nitrate:water in the ratio 1:5:10) has been used in crushed form in this application.
However, the dosage of 1% to 2% of cement weight and the generally high content of cement in mortar leads to ammonia emissions that can already exceed the odour threshold, i.e. 5-50 parts per million (ppm) of air.
Therefore, there exists the need to provide a dry mortar blend comprising calcium nitrate which does not clog or cake and in which the calcium nitrate does not precipitates when being mixed with water. A further need is to provide a dry mortar blend which is odour free when being processed.